Cabernet Franc vines with full trellis fill
providing good vine capacity, but more
vigor would give excessive canopy density.

Vine “capacity” was defined byA. J.
Winkler as the total growth of the vine
and crop; the total dry matter production
of a vine.15 This is a function of
1) seasonal sunlight intercepted by a
vine (varies with leaf area, canopy display,
and climate), 2) conversion of the
sunlight energy to dry matter (initially
sugars from photosynthesis), and 3)
the use of some sugars burned for the
energy to build vine components.

Without sacrificing entire vines, it is
not easy to measure vine capacity in
the vineyard. Instead, several indirect
measures have been developed to estimate
vine capacity.

Winter pruning weight is commonly
used as an indirect estimate of
sunlight interception and thus vine
capacity, as it is generally related to
vine size late in the season. However,
these relationships do not always hold
as stresses during the season may give
more or less vine growth and reduce
leaf function. Additionally, winter
pruning weights do not account for
shoot growth removed by summer
hedging or shoot thinning.

For a given pruning weight the
actual vine sunlight interception will
depend on spacing, training system,
canopy display, and uniformity of trellis
fill. For example, divided canopies
such as the Lyre system might have
similar leaf areas and pruning weights
to undivided canopies, but considerably
more light interception.

Additionally, the pruning weight to
light interception relationship is not
linear; changes in pruning weight at
low pruning weights will have greater
effects than at high pruning weights
where additional growth may primarily
create more shade rather than
increased light interception. Finally,
there is only one value given per year,
thus any seasonal dynamics are
ignored.

Actual leaf area attained during the
season is a more valid estimate of vine
light interception than pruning
weight, and thus the leaf area is

ine balance, often discussed and
felt to be a key goal of grapegrowing,
refers to the relative
balance of vegetative to reproductive
growth. The term crop load is
conceptually similar, but is more
restricted to the balance of vine capacity
or supply to crop demand while
not directly considering vegetative
demands.

The effects of vine balance or crop
load on wine quality are frequently
discussed in the literature and tasting
room. While there is

little debate that
there is an upper limit to how much
fruit a vine can mature before wine
quality suffers, there is also increasing
evidence that wine quality is diminished
when grapes are obtained from
vines that are under-cropped. In other
words, low quality wines are expected
from vines that are greatly out of balance,
either over-cropped or excessively
under-cropped.

Vine balance and crop load are
described in various ways that are discussed
in excellent reviews by P. Dry,
and E. Archer and J. J. Hunter.2,7 Here,
we will primarily emphasize a few
points on our current approaches,
focusing on the seasonal dynamics of
vine carbohydrate supply in relation
to crop demand, and on the relation of
vine balance to winegrape-derived
aroma compounds.